The present invention relates generally to the field of fusion welding. More particularly, but not by way of limitation, the invention pertains to a method for producing welded joints having improved low temperature properties relative to joints produced by conventional welding processes.
Frequently in industry there is a need for pressure vessels, piping, and other equipment for processing, storing, or transporting liquids or compressed gases at low temperatures. Such gases include, but are not limited to, hydrogen, helium, nitrogen, oxygen, air, or methane. In particular, it is sometimes desirable to convert a gas into liquid form for storage or transport. Natural gas is typically converted to a liquid at the extremely cold temperature of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.) and at atmospheric pressure. There is also a need for containers for storing and transporting pressurized liquefied natural gas (PLNG) at pressures in the broad range of about 1725 kPa (250 psia) to about 7590 kPa (1100 psia) and at temperatures higher than about xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.). The particular materials of construction and methods of fabrication selected for equipment for such applications depend on the operating conditions to which the equipment will be exposed.
Pressure vessels, piping, and other types of processing and storage equipment are frequently fabricated from steel alloys. As operating pressures increase, such as to 690 kPa absolute (100 psia) or more, and service temperatures decrease, such as to at or below 0xc2x0 C. (32xc2x0 F.), it becomes increasingly more difficult to achieve the required strength and fracture toughness properties with steel. Normally, stronger steel alloys are less ductile, are more susceptible to failure by brittle fracture, and are therefore not suited for cold temperature service. Conversely, steel alloys having good fracture toughness properties at lower temperatures typically have lower tensile strength and are thus not suited for high pressure applications. Therefore, as operating temperatures decrease and/or operating pressures increase, the number of steel alloys that will meet the minimum required design criteria for both strength and ductility decreases.
One means of producing the desired combination of high strength and low temperature fracture toughness in steel alloys includes proper selection of the constituent elements of the alloy and performance of particular thermal and mechanical processing steps during production of the steel. Certain combinations of constituents and thermo-mechanical processing steps produce steel alloys having specific microstructures which in turn result in the desired mechanical performance characteristics.
Fabrication of pressure vessels, piping, or other equipment usually requires the use of welded connections between steel plates, pieces of pipe, and/or other components in order to form a continuous metal enclosure. Conventional welding processes produce a heat-affected zone (HAZ) in the base metal near the fusion interface of the weld metal and the base metal. When the base metal is a steel that has been subjected to thermo-mechanical processing or other finishing steps in order to produce a specific microstructure, the heat of welding frequently results in an alteration of the microstructure and a concomitant degradation in mechanical properties. In particular, portions of the HAZ may become particularly susceptible to failure by brittle fracture. A metallurgical term that has been used to refer to small areas of low toughness within the HAZ is xe2x80x9clocal brittle zonexe2x80x9d (LBZ). Any crack in the surface of the base metal near the HAZ will have a tendency to propagate through these embrittled areas in the HAZ since the HAZ typically forms a small angle (i.e. less than 45xc2x0) with the plane perpendicular to the direction of maximum tensile load across the welded joint.
In conventional welded joints, the weld toe is defined as the region on the surface of a welded joint at the transition point between the weld metal and the base metal or alternatively as the exposed surface of the fusion interface at the welded joint. For purposes of this specification and the appended claims, a weld toe includes any exposed fusion interface, whether at the weld cap or the root of the weld, including any weld toe that is subsequently covered by another weld. The weld toe is known to be a point of high stress concentration due to both geometrical discontinuity and residual stresses from the thermal cycles of the welding process. This makes the weld toe one of the most likely sites for initiation of a crack in a welded joint. The probability of crack initiation at the weld toe and likely propagation of such a crack through multiple LBZs distributed through the HAZ limit the use of conventional welded joints in cold temperature and/or high pressure service or for welding of steel alloys having heat sensitive microstructures.
U.S. Pat. No. 4,049,186 discloses a method of reducing the probability of stress corrosion cracking in butt-welded joints in austenitic piping in nuclear reactor service. The use of various types of overlay welds on the exterior of a butt-welded joint in piping are disclosed. The purpose of these overlays is to reduce the stress on the sensitized steel at the welded joint on the inner diameter of the pipe where the sensitized steel is exposed to the process fluid. U.S. Pat. No. 5,258,600 discloses a method of connecting mechanically and/or thermally treated alloy piping. The disclosed method improves the tensile strength of the piping connection. Neither of these references addresses failure mechanisms associated with low temperature service.
U.S. Pat. No. 3,745,322 discloses a method of reducing weld bond brittleness in welded joints between high strength steels, low-temperature service steels, or low alloy steels. The method involves deposition of high notch toughness metal layers to the surfaces to be joined and subsequently applying the connecting weld to the deposited high notch toughness metal layers. The heat of application of the connecting weld also serves to heat treat the weld bond between the base metal and each high notch toughness metal layer. This process requires three separate welds for each joint thus increasing welding costs. Furthermore, the resulting HAZ""s still form a small angle with the plane perpendicular to the direction of maximum tensile load across the welded joint and are aligned with the discontinuities in the surface which are most likely to initiate cracks.
Accordingly, a need exists for welding methods that reduce the possibility of failure of a welded joint by brittle fracture, in particular by reducing the likelihood of cracks being initiated and propagating through the HAZ at the welded joint. Welded joints produced by such methods would be particularly useful in low temperature service and for welded connections between steel alloys having heat-sensitive microstructures. Preferred methods would also be suited for field applications by minimizing the importance of the orientation of the pieces to be joined and the direction of the weld in three-dimensional space.
In a preferred embodiment, the present invention relates to a method for welding together abutting pieces of steel. The pieces of steel can be plates, including juxtaposed edges of a single bent plate of steel, sections of pipe, or other formed shapes. Each piece of steel proximate the welded joint has a first surface, a second surface, and a joining surface. The pieces are positioned relative to one another prior to the welding process such that the first surfaces of the pieces of steel are substantially coextensive or aligned with one another, the second surfaces of the pieces of steel are substantially coextensive or aligned with one another, and the joining surfaces form a gap or groove suitable for application of a fusion welding process to join the two pieces of steel. After welding, these coextensive surfaces form, respectively, the first and second surfaces of a newly formed single piece of steel.
In a preferred embodiment of this invention, a strength weld is first formed between the pieces of steel. The strength weld is formed using a first weld metal and a first fusion welding process. The strength weld forms the primary load-bearing portion of the final welded joint of this invention. xe2x80x9cPrimary load-bearing portionxe2x80x9d as used in this context, means the portion that bears at least 80 percent of the load-bearing capacity of the final welded joint. A cross-section of the strength weld normal to the direction of the weld is bounded on its four sides by a first strength weld metal surface, a second strength weld metal surface opposite the first strength weld metal surface, and a first fusion interface at the junction with each piece of steel. The junction of each strength weld metal surface and each first fusion interface defines each of the four strength weld toes.
After completion of the strength weld, one or more toughness welds is formed by using a second fusion welding process to deposit a second weld metal on at least one surface of the newly formed single piece of steel covering the strength weld toes. The second weld metal covers a portion or all of at least one of the strength weld metal surfaces and a portion of the surface of each original piece of steel proximate each strength weld toe. A second fusion interface, bounded by the strength weld toe and a newly formed toughness weld toe, is created between the second weld metal and the covered portion of the surface of each piece of steel. Preferably, joining edge preparation techniques and welding methods are selected consistent with minimizing the angle formed between the direction of maximum tensile load across the welded joint and the plane containing the second fusion interface, in particular that portion of the second fusion interface adjacent the toughness weld toe. The toughness weld toe is also preferably a sufficient distance from the strength weld toe so that it does not coincide with any portion of the strength weld HAZ.
In some embodiments of the invention, the first and second weld metals are the same. In other embodiments, the first and second weld metals are different. In yet other embodiments, the first and second fusion welding processes are the same. In other embodiments, the first and second fusion welding processes are different.